1. Field of the Invention
The present invention relates to a non-heat treated steel for soft-nitriding. More specifically, it relates to a non-heat treated steel for soft-nitriding as materials of machine parts such as crankshafts or connecting rods, for example, in automobiles, industrial machines and construction machines.
2. Description of the Related Art
Heretofore, machine parts such as crankshafts or connecting rods, for example, in automobiles, industrial machines and the construction machines are manufactured by applying hot working such as hot forging and then applying thermal refining (hardening, tempering, normalizing, or annealing). The microstructure is homogenized and refined by the thermal refining. After the thermal refining, soft-nitriding is applied with an aim of improving the fatigue strength.
Since distortion occurs upon application of the soft-nitriding and deteriorates the dimensional accuracy of parts, bend leveling, or straightening, is often conducted after applying the soft-nitriding. Accordingly, it is necessary for parts after the soft-nitriding to have high fatigue strength and excellent bend leveling property.
“Excellent bend leveling property” means that cracks are not developed on the surface of the part till reaching a certain large bending displacement amount and that the fatigue strength after applying the bend leveling does not reduce so much as that before applying the bend leveling.
In the manufacture of machine parts, omission of thermal refining is desired for decreasing the manufacturing cost and saving the energy. Such demands have become strong more and more in recent years.
However, in a case where the thermal refining is omitted, non-homogeneous microstructure formed during hot working tends to remain and, further, crystal grains that are grown coarse during heating of the material before starting hot working remain in the products, which lowers the mechanical property of products. Then, normalization treatment is usually applied after hot working to solve the problem. In a case of not applying the normalization treatment after hot working, the crystal grains remain coarse, and a non-homogeneous microstructure is formed in which hot deformed structure remains partially. Accordingly, no desired fatigue strength can be obtained for the material without normalization treatment even when the soft-nitriding is applied.
Further, as described above, it is necessary for parts after soft-nitriding to have excellent bend leveling property but, in a case of omitting the thermal refining, the bend leveling property of parts after soft-nitriding is often deteriorated remarkably because of coarse crystal grain and/or non-homogeneous microstructure described above.
Accordingly, it has been demanded for the development of parts having high fatigue strength and excellent bend leveling property even in a case of omitting a thermal refining with an aim of cost reduction and energy saving, as well as a non-heat treated steel for use in soft-nitriding capable of obtaining such parts.
Then, “normalization” is to be described as a typical example of thermal refining. As the method of obtaining non-heat treated steel for soft-nitriding capable of forming parts having high fatigue strength and excellent “bend leveling property” after soft-nitriding even in a case of omitting the normalization treatment, several methods have been proposed so far. They are classified roughly into the following groups.
(1) A method of avoiding growth of the microstructure in hot forging as much as possible while keeping the microstructure of a steel to consist of ferrite and pearlite such as in thermally refined steels (for example, refer to the following Patent Documents 1 to 4).(2) A method of forming the microstructure of steel into bainite (for example, refer to the following Patent Documents 5 to 9).                [Patent Document 1] Japanese Patent Unexamined Publication No. H9-291339.        [Patent Document 2] Japanese Patent Unexamined Publication No. H9-324258.        [Patent Document 3] Japanese Patent Unexamined Publication No. H9-324241.        [Patent Document 4] Japanese Patent Unexamined Publication No. H10-46287.        [Patent Document 5] Japanese Patent Unexamined Publication No. H5-65592.        [Patent Document 6] Japanese Patent Unexamined Publication No. 2000-309846.        [Patent Document 7] Japanese Patent Unexamined Publication No. H7-157842.        [Patent Document 8] Japanese Patent Unexamined Publication No. H8-176733.        [Patent Document 9] Japanese Patent Unexamined Publication No. 2000-160287.        
The Patent Document 1 discloses a steel for nitriding in which the content of alloying elements comprises, by mass %, C: 0.15 to 0.40%, Si: ≦0.50%, Mn: 0.20 to 1.50%, Cr: 0.05 to 0.50%, and the balance Fe and inevitable impurities, in which the microstructure after hot working is substantially a ferrite—pearlite microstructure, the ferrite area fraction is 30% or more, the ferrite grain size is of 5 or more of grain size number, and the average size of pearlite is 50 μm or less. It is described that the steel is excellent in the fatigue strength and the bend leveling property after the nitriding even when normalization treatment is omitted.
The Patent Document 2 discloses nitrided parts formed by nitriding a steel in which the steel contains alloying elements comprising, by mass %, C: 0.15 to 0.40%, Si: 0.50% or less, Mn: 0.20 to 1.50%, and Cr: 0.05 to 0.50%, and the balance Fe and inevitable impurities, in which the steel has a mixed microstructure comprising ferrite and pearlite in a state as hot worked, the average size of ferrite grains is 50 μm or less, the average size of pearlite grains is 50 μm or less, the average hardening depth by the nitriding is 0.3 mm or more and the fluctuation of the hardening depth is within a range of 0.1 mm. Then, it is described that the part is excellent in the fatigue strength and the bend leveling property even in a case of nitriding while omitting the normalization treatment after hot forging.
The Patent Document 3 discloses a steel material for soft-nitriding having a chemical composition comprising, by weight %, C: 0.20 to 0.60%, Si: 0.05 to 1.0%, Mn: 0.3 to 1.0%, P: 0.05% or less, S: 0.005 to 0.10%, Cr: 0.3% or less, Al: 0.08% or less, Ti: 0.03% or less, N: 0.008 to 0.020%, Ca: 0.005% or less, Pb: 0.30% or less, Cu: 0.30% or less, Ni: 0.30% or less, Mo: 0.30% or less, V: 0.20% or less, Nb: 0.05% or less, and satisfying: 221C (%)+99.5Mn(%)+52.5Cr(%)−304Ti(%)+577N(%)+25≧150, with the balance Fe and inevitable impurities, in which the microstructure comprises ferrite and pearlite with the ferrite fraction of 10% or more, etc.
The Patent Document 3 describes that nitrided parts excellent in the fatigue strength and the bend leveling property can be obtained even when the normalization treatment is omitted in a case where the fatigue strength is expressed as the regression formulae of the contained elements and the factor is at a predetermined magnitude or more, and the microstructure comprises ferrite and pearlite with the ferrite fraction of 10% or more.
The Patent Document 4 discloses a steel for nitriding comprising, by weight %, C: 0.30 to 0.43%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.60%, P: 0.08% or less, S: 0.10% or less, sol. Al: 0.010% or less, Ti: 0.013% or less, Ca: 0.0030% or less, Pb: 0.20% or less, and N: 0.010 to 0.030%, and the balance Fe and impurities, in which Cr is 0.10% or less and V is 0.01% or less in the impurities, etc.
The Patent Document 4 describes that a product excellent in the fatigue strength and the bend leveling property can be obtained by moderating the hardness gradient in a nitriding layer even when applying nitriding while omitting the normalization treatment.
The Patent Document 5 discloses a steel with high fatigue strength comprising, C: 0.1 to 0.35%, Si: 0.05 to 0.35%, Mn: 0.6 to 1.50%, P: 0.01% or less, S: 0.015% or less, Cr: 1.1 to 2.0%, Mo: 0.5 to 1.0%, V: 0.03 to 0.13%, B: 0.0005 to 0.0030%, Ti: 0.01 to 0.04%, Al: 0.01 to 0.04%, and the balance Fe and inevitable impurities, etc.
The Patent Document 5 describes that Cr is effective for improving the hardenability and nitriding hardenability and V is effective for refining precipitated carbides to enhance the fatigue strength. In this case, since the nitriding hardenability caused by Cr is due to precipitation of Cr nitrides, improvement in the fatigue strength in this case is based on precipitation hardening by Cr and V. However, in the Patent Document, a once produced steel material is again heated and cooled to form a bainite microstructure, then the steel is classified into the category of thermally refined steel.
The Patent Document 6 discloses a non-heat treated steel for soft-nitriding containing, by mass %, C: not less than 0.1% but less than 0.3%, Si: 0.01 to 1.0%, Mn: 1.5 to 3.0%, Cr: 0.01 to 0.5%, Mo: 0.1 to 1.0%, acid soluble Al: 0.01 to 0.045%, N: 0.005 to 0.025%, and the balance Fe and inevitable impurities, etc.
The Patent Document 6 describes that the steel having the bainite structure obtained by air cooling from the hot working temperature is excellent in toughness and has excellent bend leveling property after applying soft-nitriding. In this case, the C content is defined as less than 0.3% in order that the hardness of bainite does not become excessive to deteriorate the machinability, and the Mn content is defined as 1.5% or more for ensuring the hardenability of the steel to form bainite. Further, the hardness of the nitrided layer is intended to be increased by precipitation hardening due to Cr nitrides by the addition of 0.01 to 0.05% of Cr. That is, in the Patent Document, the C content is defined as less than 0.3% so that the hardness of bainite is not excessively high, on the bases of the fact that the bend leveling property is improved by the bainite microstructure because bainite has higher toughness than the ferrite-pearlite microstructure at an identical hardness. However, a steel where the C content is less than 0.3% is worried about the lack of wear resistance. In machine parts such as crankshafts and connecting rods, the wear resistance is also an extremely important factor.
The Patent Document 7 discloses a steel for soft-nitriding having a chemical composition comprising, by weight %, C: 0.05 to 0.30%, Si: 1.20% or less, Mn: 0.60 to 1.30%, Cr: 0.70 to 1.50%, Al: 0.10% or less, N: 0.006 to 0.020%, V: 0.05 to 0.20%, Mo: 0 to 1.00%, B: 0 to 0.0050%, S: 0 to 0.060%, Pb: 0 to 0.20%, Ca: 0 to 0.010%, satisfying 0.60≦C+0.1Si+0.2Mn+0.25Cr+1.65V≦1.35, or satisfying 0.60≦C+0.1Si+0.2Mn+0.25Cr+1.65V+0.55Mo+8B≦1.35, and the balance Fe and inevitable impurities, with the hardness of a core part of Hv 200 to 300 and the microstructure being a bainite or having a mixed microstructure of “ferrite+bainite” where ferrite fraction is less than 80%, by cooling after hot rolling or hot forging with no heat treatment.
The invention in the Patent Document 7 also adopts the idea of improving the fatigue strength by utilizing the precipitation hardening caused by Cr and V as well as in Japanese Patent Unexamined Publication No. H5-65592 described above. However, since the C content is defined as less than 0.3%, the worry about the lack of the wear resistance remains as well as in Japanese Patent Unexamined Publication No. 2000-309846 described above.
The Patent Document 8 discloses a steel for soft-nitriding comprising, by weight %, C: 0.15 to 0.40%, Si: 1.20% or less, Mn: 0.60 to 1.80%, Cr: 0.20 to 2.00%, Al: 0.02 to 0.10%, N: 0.006 to 0.020%, V: 0.05 to 0.20%, and the balance Fe and inevitable impurities, and satisfying both conditions of 0.60≦C+0.1Si+0.2Mn+0.25Cr+1.65V≦1.35 and 0.25Cr+2V≦0.85, with the hardness for a core part of Hv 200 to 300, and having mixed microstructure of “ferrite+pearlite” or a mixed microstructure of “ferrite+pearlite+bainite whose bainite fraction is less than 20%”, by cooling after hot rolling or hot forging without heat treatment, and discloses a steel that has high surface hardness and deep hardened depth and, further, low heat treatment distortion by applying soft-nitriding.
It is expected that the disclosed steel in the Patent Document 8 is improved for the wear resistance since the C content is 0.15 to 0.40%. However, the idea of improving the fatigue strength by utilizing the precipitation hardening caused by Cr and V is also adopted in this steel as well as in the invention of Japanese Patent Unexamined Publication No. H7-157842 described above.
The Patent Document 9 discloses a non-heat treated forged nitrided parts containing, C: 0.15 to 0.35%, Mn: 1.00 to 3.00%, Cr: 0 to 0.15%, V: 0 to 0.02%, Cu: 0.50 to 1.50%, and Ni: 0.4 times or more of the Cu content, with B, N and Ti contents satisfying 0.0010 to 0.0030% of Bsol as defined by Bsol=B−(11/14){N−(14/48) Ti}, with the balance Fe and inevitable impurity.
The Patent Document 9 describes as follows:
It is preferred that the steel for soft-nitriding comprises a ferrite-based microstructure or, in a case where it is difficult, a single phase microstructure of martensite or bainite rather than a mixed microstructure of “ferrite+pearlite”. This adopts an idea of utilizing precipitation hardening caused by Cu instead of by Cr and V. Further, it is described that Mn content has to be 1.0% or more in order to obtain the fully bainitic microstructure, which means to intend to a non-heat treated steel with fully bainitic microstructure.
As has been described above, it has been already known to obtain a non-heat treated steel utilizing the bainite microstructure for soft-nitriding, which provides parts excellent in the fatigue strength and the bend leveling property after soft-nitriding. However, improvement of the fatigue strength by the precipitation hardening caused by the alloying additions deteriorates the bend leveling property on the other hand. That is, the subject of compatibilizing the high fatigue strength and the excellent bend leveling property has not yet been solved.
Further, in order to cope with the demand of further increasing the strength of parts in recent years, it has been demanded for a non-heat treated steel for soft-nitriding, which provides soft-nitrided parts that have higher fatigue strength and are excellent in the bend leveling property. However, the existent technique of “precipitation hardening and bainitic microstructure” cannot always cope with such a demand.